CILIATES OF THE FROG 6l 



More numerous and conspicuous than either of these is Opalina 

 rananim, a flat, oval, pale-straw-coloured ciliate of very large 

 size (i mm. long), uniformly covered with equal cilia, and without 

 mouth, peristome, contractile vacuole, or trichocysts. It has many 

 nuclei, unlike those of other Ciliata in being all of one kind. The 

 life-history is also very unlike that of other Ciliata. Nuclei and 

 cytoplasm divide independently (the latter alternately in a 

 longitudinal and a transverse direction), and during the greater 

 part of the year keep pace with one another and with growth, so 

 that the appearance of the mature animals remains the same ; 

 but in the spring the division of the cytoplasm gains, so that 

 small individuals with 3-6 nuclei result. It is said that at this 

 time a portion of the chromatin of the nuclei passes in granular 

 or ' chromidial ' form into the cytoplasm, where it perishes. The 

 little individuals now encyst. The cysts are passed by the frog 

 into the water and there swallowed by tadpoles, in which they 

 hatch, and their cytoplasm divides to form uninuclear gametes, 

 the nuclei meanwhile undergoing a reducing division. The gametes 

 conjugate, and the zygote encysts. Probably it always at this 

 stage passes out of the host and enters another, where it hatches. 

 From the cyst emerges a uninuclear ciliate which grows into the 

 adult. Daring the whole process cilia are lost only in the zygote 

 cyst. 



